Flexible wing-sail and wind-propelled vehicle including same

ABSTRACT

A flexible wing-sail for wind-propelled vehicles includes a mast to be rotatably mounted in a vertical position on the vehicle, a plurality of flexible sail panels carried by the mast, and a spreader assembly secured to battens in the sail panels for securing them to the mast and for imparting to them an airfoil shape having a leading edge fore of the mast, and a trailing edge aft of the mast. The spreader assembly includes a fixed spreader unit secured to the sail panels at the bottom of the wing-sail and fixing it to the bottom of the mast, and a plurality of slidable spreader units secured to the sail panels at longitudinally spaced locations and slidable along the mast to permit hoisting and reefing the wing-sail. The wing-sail further includes a boom pivotally coupled to the bottom of the mast and having a sliding coupling with respect to the sail panels at the trailing edge of the wing-sail, and a brake for selectively locking the mast against rotation, such that pivoting the boom while the mast is locked, changes the curvature of the airfoil shape defined by the sail panels.

FIELD AND BACKGROUND OF THE INVENTION

[0001] The present invention relates to flexible wing-sails and towind-propelled vehicles including flexible wing-sails. The invention isespecially useful in wind-propelled water vehicles, and is thereforedescribed below with respect to such application, but it will beappreciated that the invention could also be used in other applications,such as for propelling vehicles over land or ice.

[0002] The conventional water vehicle sail functions in most winddirections like a wing of lower aerodynamic efficiency than an aircraftwing because the sail is a single sheet of fabric and also because thesail form is dictated by the vehicle structure. The conventional sailincludes a fixed mast, fixed points at which the sail is anchored to thevehicle, and a single-surface structure. These features do not enablesuch a sail to have high aerodynamic efficiency, but rather result inits having a lower lift/drag ratio than that of an aircraft wing.

[0003] In recent years, a number of wing-sails have been proposed foruse in water vehicles in order to better exploit the wind forces forpropelling the vehicle. Basically, a wing-sail includes two curvedsurfaces defining a wing which is relatively thick and rounded at itsleading edge, and tapers in thickness to its trailing edge. When therounded, leading edge of an asymmetrical wing-sail is oriented to facethe wind, the difference in air pressure between its two curved surfacescreates a lifting force which, in the case of wind-driven vehicles, istranslated to a forward propulsion force. Examples of variousconstructions of wing-sails heretofore proposed are described in U.S.Pat. Nos. 4,685,410; 4,733,624; 4,856,449; 4,895,091; 5,406,902;5,575,233; 5,622,131; and 6,141,809, and in U.K. Patents 2,008,514; and2,196,310.

[0004] However, the proposed solutions to the problem generally werepartial only. They included the option of a rotating mast that carrieswith it the usual rigging and sails as well as a rigid wing that cannotbe reefed. Where a soft wing-sail was proposed permitting reefing, thewing-sail had an airfoil shape which is symmetric, wholly or partly andtherefore was not sufficiently efficient. Several suggested solutionsproposed an asymmetric airfoil, but the departures from symmetry arelimited to a movable surface in the rear part of the wing-sail. Othersolutions that were suggested are limited in flexibility, or are socomplex that it is doubtful whether they could function under marineconditions. In most suggested solutions, the asymmetric variations arelimited to two positions only (port/starboard), without control of thecurvature of the airfoil shape of the wing-sail.

[0005] There is therefore a definite need for a flexible wing-sailconstruction providing increased aerodynamic efficiency, capable ofbeing reefed and taken down, of being pointed to the wind, of enablingchanges of the airfoil shape to either port or starboard in accordancewith the apparent wind direction, and of enabling changes in the rate ofasymmetry of the airfoil shape in accordance with the apparentwindforce. Such higher efficiency would enable the attainment of higherspeeds of travel, or alternatively, a reduction in the size of thesails, rigging and keel. It would also enable sailing a vessel moreclosely to the wind, less heeling, and more convenience in operatingwind-driven vehicles, not only water vehicles, but also land and icevehicles.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

[0006] An object of the present invention is to provide a flexiblewing-sail for a wind-propelled vehicle having a number of advantages inthe above respects as will be described more particularly below.

[0007] According to one aspect of the present invention, there isprovided a flexible wing-sail for wind-propelled vehicles, comprising: amast to be rotatably mounted in a vertical position on the vehicleenabling the flexible wing-sail to freely rotate and assume paralleldirection to the apparent wind; a plurality of flexible sail panelscarried by the mast and including first, second and third sail panelsdefining the opposite sides and the trailing edge of the flexiblewing-sail, and a third sail panel defining the leading edge of theflexible wing-sail; a spreader assembly securing the sail panels to themast and imparting to the sail panels an airfoil shape having a leadingedge fore of the mast, and a trailing edge aft of the mast; a boompivotally coupled to the bottom of the mast above deck and including aslidable coupling to the trailing edge of each of the first and secondsail panels; and a brake for selectively locking the mast againstrotation with respect to the vehicle, such that pivoting the boom whilethe mast is locked against rotation, changes the curvature of theairfoil shape defined by the first and second sail panels according tothe apparent wind direction and velocity.

[0008] According to further features in the described preferredembodiment, the trailing edge of each of the first and second sailpanels is slidably coupled to the boom by a slide movable within a slotin the boom and urged by a spring or by an elastic line outwardly of theboom away from the pivotal coupling of the boom to the mast.

[0009] According to another aspect of the present invention, there isprovided a flexible wing-sail for wind-propelled vehicle, comprising: amast to be rotatably mounted in a vertical position on the vehicle; aplurality of flexible sail panels carried by the mast; and a spreaderassembly securing the flexible sail panels to the mast and imparting tothe flexible sail panels an airfoil shape having a leading edge fore ofthe mast, and a trailing edge aft of the mast; the spreader assemblyincluding a fixed spreader unit secured to the flexible sail panelsdefining the bottom of the flexible wing-sail and fixing it to thebottom of the mast, and a plurality of slidable spreader units securedto the flexible sail panels at longitudinally spaced locations thereofand slidable along the mast to permit hoisting and reefing the flexiblewing-sail.

[0010] According to further features in the described preferredembodiment, the fixed spreader unit is secured only to the sail panelsat the leading edge of the flexible wing-sail and fixes them to thebottom of the mast, and the slidable spreader units are secured only tothe sail panels at the leading edge of the flexible wing-sail andslidable mount them to the mast.

[0011] According to yet another aspect of the present invention, thereis provided a flexible wing-sail for wind-propelled vehicles,comprising: a mast to be rotatably mounted in a vertical position on thevehicle; a plurality of flexible sail panels carried by the mast; and aspreader assembly securing the flexible sail panels to the mast andimparting to the flexible sail panels an airfoil shape having a leadingedge fore of the mast, and a trailing edge aft of the mast; the flexiblesail panels being secured to the spreader assembly by battens receivedin pockets in the flexible sail panels.

[0012] As will be described more particularly below, such a constructionpermits the flexible wing-sail to be hoisted, reefed, lowered, orotherwise adjusted in accordance with the apparent wind direction andwind velocity. The higher efficiency capability of such a flexiblewing-sail enables the vehicle to attain higher speeds of travel, oralternatively, to reduce the size of the sails and rigging. It enablesmore convenient operation of a vehicle driven by wind, and also enablessailing the vehicle more closely against the wind. In addition, itenables such advantages to be attained by a flexible wing-sail of arelatively simple construction.

[0013] Further features and advantages of the invention will be apparentfrom the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

[0015]FIG. 1 illustrates one form of water vehicle equipped with aflexible wing-sail in accordance with the present invention;

[0016]FIG. 2 is a fragmentary view illustrating the rotary mounting ofthe mast in the vehicle of FIG. 1;

[0017]FIG. 3a is a fragmentary view illustrating the slidable mountingof the spreader units to the mast in the vehicle of FIG. 1;

[0018]FIG. 3b is an enlarged fragmentary view of a portion of FIG. 3a;

[0019]FIG. 4 is a horizontal sectional view of the flexible wing-sail inthe vehicle of FIG. 1 to illustrate the structure of the slidablespreader units;

[0020]FIG. 5 is a top view illustrating the top spreader unit in theflexible wing-sail of FIG. 1;

[0021]FIG. 6 is a fragmentary view of the upper end of the flexiblewing-sail in FIG. 1;

[0022]FIG. 7 is a side elevational view of the flexible wing-sail inFIG. 1;

[0023]FIG. 8 is a top view illustrating another construction of slidablespreader unit that may be used in the flexible wing-sail;

[0024]FIG. 9 is a view similar to that of FIG. 8, but illustrating astill further construction of slidable spreader unit that may be used;

[0025]FIG. 10a is a top view illustrating a modification in theconstruction of the top spreader unit that may be used;

[0026]FIG. 10b is a fragmentary view illustrating the upper end of theflexible wing-sail when using the top spreader unit shown in FIG. 10a;

[0027]FIGS. 11a and 11 b illustrate the manner in which asymmetricalcurvatures to either port or starboard may be applied to the flexiblewing-sail of FIG. 1;

[0028]FIGS. 12a, 12 b and 12 c diagrammatically illustrate the manner inwhich the curvature of the airfoil may be changed according to theapparent wind direction and force;

[0029]FIG. 13 diagrammatically illustrates a flexible wing-sailconstructed in accordance with the present invention equipped with motordrives for manually or automatically controlling the orientation and/orcurvature of the airfoil according to apparent wind conditions; and

[0030]FIG. 14 is a block diagram illustrating a control system which maybe used for controlling the flexible wing-sail of FIG. 13 according tothe apparent wind conditions.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0031]FIG. 1. illustrates a water vehicle including a hull 2 having avertically-extending mast 3 supporting a flexible wing-sail 4constructed in accordance with the invention. The mast 3 mounts theleading edge 4 a of the flexible wing-sail 4, whereas the trailing endof the flexible wing-sail is coupled to a boom 5 which is pivotallymounted at 6 to the mast 3. As will be described more particularlybelow, the flexible wing-sail 4 is made of a plurality of flexible sailpanels carried by the mast 3, and includes a spreader assembly spreadingthe flexible sail panels and securing them to the mast. The spreaderassembly imparts to the flexible sail panels an airfoil shape havingleading edge 4 a fore of the mast, and a trailing edge 4 b aft of themast.

[0032] The flexible wing-sail may be hoisted or lowered by a pair ofhalyards 7 coupled to the upper end of the flexible wing-sail. Thebottom end of the flexible wing-sail 4 is attached to the bottom part ofthe spreader assembly, which in turn is rigidly attached to the mast 3.A bottom spar 8 braces the leading edge of the flexible wing-sail 4, andanother bottom spar 9 braces the boom 5.

[0033] The mast 3 is rotatably mounted to the hull 2. Thus, as shown inFIG. 2, the mast 3 is rotatably mounted between a rotary bearing 10carried by the bottom 11 of the hull 2, and by a second rotary bearing12 carried by the deck 13 of the hull.

[0034] As distinguished from conventional constructions, in this casethe rotary mast 3 may be selectively locked against rotation by means ofa brake or other locking device. FIG. 2 illustrates one example of sucha locking device including a disc 14 secured to the mast 3 and having anouter edge received within the locking device 15 secured to the vehicledeck 13. The locking device 15 shown in FIG. 2 includes a pair ofpressure plates 15 a, 15 b selectively actuated by an actuator 16 tofirmly engage the disc 14 in order to lock the mast 3 against rotation,or to disengage the disc in order to permit the mast to freely rotate.Actuator 16 may be hydraulically or pneumatically actuated via a controlline 17. It will be appreciated that other types of locking devices maybe used.

[0035] The flexible wing-sail 4 carried by the mast 3 is constructed ofa plurality of flexible sail panels, as will be described moreparticularly below with respect to FIGS. 4 and 5. The sail panels attheir leading ends are spread apart by a spreader assembly including aplurality of vertically-spaced spreader units, one of which is shown at20 in FIGS. 3 and 4. As shown particularly in FIG. 4, battens 21,received in pockets in the flexible wing-sail 4, are secured to theouter ends of each spreader unit 20. Each spreader unit 20 furtherincludes a slide 22 slidable along rails 23 fixed to the opposite sidesof the mast 3.

[0036] It will thus be seen that, by appropriately manipulating thehalyard 7 (FIG. 1), the slidable couplings between the spreader units 20and the rails 23 of the mast 3, permit the flexible wing-sail 4 to bedeployed to a fully hoisted position, as shown in FIG. 1, to a fullyreefed position, or to any partially reefed position in between.

[0037] FIGS. 4-7 illustrate the construction of the flexible wing-sail4. It is made of flexible sail panels each of sailcloth formed withpockets for receiving the battens 21 which support and shape theflexible wing-sail.

[0038] Thus, as shown in FIG. 5, flexible wing-sail 4 is constructed ofthree flexible sail panels, shown in broken lines at 26, 27 and 28 eachformed with the pockets for the battens 21. Flexible panel 26 extends onone side to the trailing edge 4 b of the flexible wing-sail 4; flexiblepanel 27 extends on the opposite side to the trailing edge of theflexible wing-sail; and flexible panel 28 joins the leading ends of thetwo flexible panels 26, 27 to define the leading edge 4 a of theflexible wing-sail 4.

[0039] Flexible panels 26, 27, 28 may be constituted of a single sheet.Alternatively, they may be made of three separate panels sewn togetherat their respective edges and attached to the spreader units 20 byattaching their battens 21 to the ends of the spreader units.

[0040] Preferably, the battens 21 in sail panel 28 defining the leadingedge 4 a of the flexible wing-sail 4 are stiffer than the battens 21 inthe two sail panels 26, 27 defining the sides of the flexible wing-sail.As will be described more particularly below, the rounded configurationof the leading edge 4 a of the flexible wing-sail remains substantiallythe same under all wind conditions, whereas the sail panels defining thetwo sides of the flexible wing-sail to the trailing edge 4 b of theflexible wing-sail do change in curvature according to the apparent windconditions. For example, the battens 21 within the leading edge sailpanel 28 may be steel or plastic rods, whereas the battens in the sailpanels 26 and 27 may be fiber strands.

[0041]FIG. 5 illustrates one option wherein the top spreader unit 20 atthe upper end of the flexible wing-sail 4 includes atransversely-extending section 20 a which, together with anaxially-extending section 20 b, produces a thick, rounded, leading edge4 a to the flexible wing-sail. The effective thickness of the airfoildecreases from the transverse spreader unit 20 towards the trailing edge4 b of the airfoil as shown particularly in FIGS. 11a and 11 b. As alsoshown in FIGS. 11a and 11 b, the bottom of the trailing edge of sailpanel 26 terminates in a slide 31 received within a slot 31 a in theboom 5; and the bottom of the trailing edge of sail panel 27 terminatesin a slide 32 received within a slot 32 a in the boom. Slide 31 of sailpanel 26 is urged outwardly of the boom 5, that is, away from itspivotal mounting 6 to the mast, by an elastic line or a spring member34; and similarly, slide 32 of sail panel 27 is urged outwardly of theboom by an elastic line or a spring member 35. As will be describedbelow particularly with respect to the description of FIGS. 11a, 11 band FIGS. 12a-12 c, such a construction permits the asymmetric shape ofthe airfoil to be changed, as desired, according to apparent windconditions.

[0042]FIG. 6 illustrates the manner of hoisting and reefing the flexiblewing-sail illustrated in FIG. 5. Thus, one end of the halyards 7 wouldbe secured at points 20 c to the opposite ends of the axial section 20 bof the topmost spreader unit 20. The opposite ends of the halyards 7 arethen passed around rollers 42 carried by a top plate 33 at the upper endof the mast 3. The latter ends are grasped by the user such that pullingthe halyards hoists the flexible wing-sail, whereas releasing thehalyards permits the flexible wing-sail to be reefed by gravity.

[0043]FIG. 8 illustrates a flexible wing-sail of a similar constructionas described above, except that the spreader unit, therein designated20′, is of a slightly different configuration than spreader unit 20shown in FIGS. 4 and 5. Each spreader unit 20′ in FIG. 8 is also of aone part construction, being slidably mounted at its center on the mast3′; it also, extends transversely of the leading edge of the flexiblewing-sail 4′, joining the leading edges of the two flexible panels 26′,27′, to the rounded panel 28′ at the leading edge of the flexiblewing-sail.

[0044]FIG. 9 illustrates a slightly different construction wherein eachof the spreader units is made of two sections, shown at 20 a″, 20 b″,respectively. One end of each section joins the rounded sail panel 28″at the leading edge of the flexible wing-sail to the respective sailpanels 26″, 27″. The opposite end of each section is slidably mounted tothe mast 3″. In all other respects, the flexible wing-sail illustratedin FIG. 9 is constructed and is used in the same manner as describedabove.

[0045]FIGS. 10a and 10 b illustrate the option of mounting the halyards7 directly to the upper end of the flexible wing-sail to fully hoist it,to fully reef it, or to partially reef it. Thus one end of each halyard7 is coupled to a tab 41 secured to the top of the respective sail paneland passes around a roller 42 fixed to the top plate 33 at the upper endof the mast 3, such that the halyard may be pulled to hoist the flexiblewing-sail, or released to partially or fully reef it. As shown in FIG.1, the trailing end of the flexible wing-sail may be provided with aplurality of tabs 41 along its height, to enable the flexible wing-sailto be secured in a partially reefed condition.

[0046]FIGS. 11a, 11 b illustrate the manner in which the flexiblewing-sail 4 may be formed into a desired asymmetric airfoil shapeaccording to the apparent wind direction. For this purpose the aft endof the boom 5 is provided with a boom line 40 which may be pulledtowards the hull axis, in order to pivot the boom 5 with respect to themast 3, when the flexible wing-sail direction is parallel to the windand the mast is locked in place by the locking device 15 (FIG. 2). Thus,as shown in FIGS. 11a and 11 b, when the mast is locked againstrotation, pulling boom line 40 to pivot the boom 5 towards the hull axiswill cause slides 31 and 32, at the trailing edges of the two sailpanels 26, 27, to slide within their respective slots 31 a, 32 a of theboom, thereby enabling the curvature of the two sail panels 26, 27 to beincreased; whereas releasing the boom line to permit the boom to pivotaway from the hull axis by the wind force, will cause the slides 31, 32to move in their respective slots, thereby decreasing the curvature ofthe sail panels up to a symmetrical airfoil configuration.

[0047] The battens 21 are preferably of varying thickness, being thickerat the front end of the sail panels 26, 27, than at the trailing end 4b. Each batten is attached at its front to the respective spreader unit20, and at its rear to the respective sail panel 26, 27. Thus, when thewind is in the direction shown in FIG. 11a and 11 b and the flexiblewing-sail direction is parallel to the wind, pivoting the boom 5 towardsthe hull axis causes, in both cases, the sail panel on the leeward sideto assume a convex form, and the sail panel on the windward side toassume a concave form. In this way, the airfoil shape of the flexiblewing-sail assumes an asymmetric form in accordance with the winddirection, whether the wind is from the port or from the starboard side.

[0048]FIGS. 12a-12 c illustrate the deployment of the flexible wing-sail4 under varying wind conditions.

[0049] In FIG. 12a, the wind is of high velocity producing a high windforce applied to the flexible wing-sail. In this case, the mast lock 15(FIG. 2) would be released so that the mast 3 would freely rotate toenable the flexible wing-sail to assume the direction of the wind, asshown in FIG. 12a. In this case, the axis of the boom 5 is perpendicularto the axis of the mast 3, so that the flexible wing-sail 4 assumes asymmetrical airfoil shape.

[0050] It will be appreciated that, in a high wind condition, the sailmay be partially reefed or fully reefed in order to decrease the windforce applied to the flexible wing-sail, although actually, the drag ofthe airfoil produced by a hoisted wing-sail is less than the dragproduced by the mast itself in a fully reefed wing-sail.

[0051]FIG. 12b illustrates the condition wherein there is a medium windforce. In this case, the mast 3, after assuming the wind direction,would be locked against rotation, and the boom line 40 would be pulledto pivot the boom 5 towards the hull axis. This will cause the airfoilto become asymmetric in shape and the angle of attack to increase, asdescribed above with respect to FIGS. 11a and 11 b and as shown in FIG.12b, which thereby increases the aerodynamic forces applied by the windto the flexible wing-sail.

[0052]FIG. 12c illustrates the condition wherein there is very low windvelocity. In this case, the boom 5 is pivoted to a greater extenttowards the axis of the hull while the mast is locked against rotation.This increases the curvature of the airfoil as well as the angle ofattack, and thereby increases the aerodynamic forces applied by the windto the flexible wing-sail.

[0053] The flexible wing-sail as described above may be controlled asalready described without recourse to any mechanically drives. When sucha flexible wing-sail is to be implemented in a larger vessel, and/orwhen automation is desired, the system may include electric or hydraulicmotors to control the various operations described above.

[0054]FIG. 13 diagrammatically illustrates a motorized control that maybe used for the various control operations; and FIG. 14 diagrammaticallyillustrates a control system which may be used for either manual controlor automatic control.

[0055] Thus, as shown in FIG. 13, the mast 3 carrying the flexiblewing-sail 4 may be rotated by a motor M₁ via a gear 106 driven by themotor and meshing with a gear 107 fixed to the mast. As also shown inFIG. 13, the boom 5 may be pivoted towards or away from the hull axis bya second motor M₂ driving a nut 108 with respect to a screw 109 coupledto the boom.

[0056] The vehicle may also include a third motor M₃ (FIG. 14) coupledto the halyards (e.g., 7, FIG. 1) for raising and lowering the flexiblewing-sail.

[0057]FIG. 14 illustrates a control system, generally designated 110,which may be operated according to either a manual mode or an automaticmode, as may be selected by a mode selector 111. When the manual mode isselected, the rotary position of the mast may be controlled by manualcontrol device 112 which controls motor M₁ to rotate the mast; and thedegree of curvature of the airfoil may be selected by manual controldevice 113 which controls motor M₂ to pivot the boom. The deployment ofthe flexible wing-sail may also be controlled by a manual control device114 which controls motor M₃ to hoist or reef the flexible wing-sail.

[0058] On the other hand, when the automatic control is selected by themode selector 111, motor M₁ which rotates the mast is automaticallycontrolled by a wind direction sensor 115 to maintain the flexiblewing-sail direction parallel to the apparent wind; and motor M₂ isautomatically controlled in response to a wind velocity sensor 116 tochange the angle of the boom with respect to the mast, and thereby theasymmetric curvature of the airfoil, in order to maintain the optimumairfoil shape in accordance with the apparent wind force.

[0059] It will thus be seen that when mode selector 111 of thecontroller 110 selects the automatic mode, the flexible wing-sailautomatically turns into the right direction relative to the apparentwind, and at the same time, the airfoil shape is automatically adjustedto the right direction and the right degree of curvature so as toproduce optimum aerodynamic efficiency.

[0060] A wind-driven vehicle constructed in accordance with theforegoing features of the invention thus provides the skipper with fullcontrol on everything required from a wing-sail. The effective surfacearea of the flexible wing-sail may be controlled by means of thehalyards 7 to permit reefing as in a conventional sail; the direction ofthe flexible wing-sail may be controlled by releasing the mast brakedevice 15, letting the flexible wing-sail to spontaneously rotate andassume direction with respect to the apparent wind, and relock the mastbrake device 15. In order to adjust the flexible wing-sail to theapparent wind force, the degree of asymmetry of the airfoil shape may becontrolled by changing the angle between the boom 5 and the mast 3. Thevehicle may include simple controls as described above, or the controlsmay be automated by a control system as also described above.

[0061] While the invention has been described above with respect towind-driven water vehicles, it will be appreciated that the inventioncould also be implemented in wind-driven land vehicles or ice vehicles.Many other variations, modifications and applications of the inventionwill be apparent.

What is claimed is:
 1. A flexible wing-sail for wind-propelled vehicles,comprising: a mast to be rotatably mounted in a vertical position on thevehicle enabling the flexible wing-sail to freely rotate and assumeparallel direction to the apparent wind; a plurality of flexible sailpanels carried by said mast and including first, second and third sailpanels defining the opposite sides and the trailing edge of the flexiblewing-sail, and a third sail panel defining the leading edge of theflexible wing-sail; a spreader assembly securing said sail panels to themast and imparting to the sail panels an airfoil shape having a leadingedge fore of the mast, and a trailing edge aft of the mast; a boompivotally coupled above deck to the bottom of said mast and including aslidable coupling to the trailing edge of each of said first and secondsail panels; and a brake for selectively locking the mast againstrotation with respect to the vehicle, such that pivoting the boom whilethe mast is locked against rotation, changes the curvature of theairfoil shape defined by the first and second sail panels according tothe apparent wind direction and velocity.
 2. The flexible wing-sailaccording to claim 1, wherein the trailing edge of each of said firstand second sail panels is slidably coupled to said boom by a slidenormally urged by an elastic element rearwardly of the boom away fromthe pivotal coupling of the boom to the mast.
 3. The flexible wing-sailaccording to claim 1, wherein said spreader assembly includes a fixedspreader unit secured to the flexible sail panels defining the bottom ofthe flexible wing-sail and fixing it to the bottom of the mast, and aplurality of slidable spreader units secured to the flexible sail panelsat longitudinally spaced locations thereof and slidable along said mastto permit hoisting and reefing the flexible wing-sail.
 4. The flexiblewing-sail according to claim 3, wherein said fixed spreader unit issecured only to the sail panels at the leading edge of the flexiblewing-sail and fixes them above deck to the bottom of the mast, and saidslidable spreader units are secured only to the sail panels at theleading edge of the flexible wing-sail and slidable mount them to themast.
 5. The flexible wing-sail according to claim 1, wherein saidfirst, second and third sail panels include battens received in pocketsin the respective sail panel, and wherein said spreader units aresecured to said battens at the joined edges of the sail panels.
 6. Aflexible wing-sail for wind-propelled vehicles, comprising: a mast to berotatably mounted in a vertical position on the vehicle; a plurality offlexible sail panels carried by said mast; and a spreader assemblysecuring said flexible sail panels to the mast and imparting to theflexible sail panels an airfoil shape having a leading edge fore of themast, and a trailing edge aft of the mast; said spreader assemblyincluding a fixed spreader unit secured to the flexible sail panelsdefining the bottom of the flexible wing-sail and fixing it to thebottom of the mast, and a plurality of slidable spreader units securedto the flexible sail panels at longitudinally spaced locations thereofand slidable along said mast to permit hoisting and reefing the flexiblewing-sail.
 7. The flexible wing-sail according to claim 6, wherein saidfixed spreader unit is secured only to the sail panels at the leadingedge of the flexible wing-sail and fixes them to the bottom of the mast,and said slidable spreader units are secured only to the sail panels atthe leading edge of the flexible wing-sail and slidable mount them tothe mast.
 8. The flexible wing-sail according to claim 7, wherein saidplurality of sail panels include first and second sail panels definingthe opposite sides and the trailing edge of the flexible wing-sail, anda third sail panel defining the leading edge of the flexible wing-sail.9. The flexible wing-sail according to claim 8, wherein the flexiblewing-sail further includes a boom pivotally coupled to the bottom ofsaid mast, the trailing edge of each of said first and second sailpanels being slidably coupled to said boom.
 10. The flexible wing-sailaccording to claim 9, wherein the mast includes a brake for selectivelylocking the mast against rotation with respect to the vehicle, such thatpivoting the boom while the mast is locked against rotation, changes thecurvature of the airfoil shape defined by the first and second sailpanels.
 11. The flexible wing-sail according to claim 10, wherein thetrailing edge of each of said first and second sail panels is slidablycoupled to said boom by a slide normally urged rearwardly of the boomaway from the pivotal coupling of the boom to the mast.
 12. The flexiblewing-sail according to claim 11, wherein said slide is movable within aslot in said boom and is urged by a spring or by an elastic linerearwardly of the boom away from the pivotal coupling of the boom to themast.
 13. The flexible wing-sail according to claim 8, wherein saidfirst, second and third sail panels include battens received in pocketsin the respective sail panel, and wherein said spreader units aresecured to said battens at the joined edges of the sail panels.
 14. Theflexible wing-sail according to claim 13, wherein said battens in thepockets of the third sail panel are stiffer than those in the pockets ofsaid first and second sail panels.
 15. The flexible wing-sail accordingto claim 8, wherein each of said slidable spreader units is slidablymounted at its center to said mast, and is secured at one end to theedge of the first sail panel joined to the third sail panel, and at itsopposite end to the edge of the second sail panel joined to the thirdsail panel.
 16. The flexible wing-sail according to claim 8, whereineach of said slidable spreader units includes a first section slidablymounted at one end to said mast and at its opposite end to the edge ofthe first sail panel joined to the third sail panel, and a secondsection slidably mounted at one end to said mast and at the opposite endto the edge of the second sail panel joined to the third sail panel. 17.A wind-propelled vehicle including a flexible wing-sail according toclaim 1 and further including: an apparent wind direction and wind forcesensor; a motor for selectively rotating said mast; and a control systemhaving: (i) a manual mode of operation, wherein said motor is manuallycontrolled to change the direction of the flexible wing-sail withrespect to the apparent wind direction, and (ii) an automatic mode ofoperation, wherein said motor is automatically controlled in response tothe apparent wind direction, as sensed by said wind direction sensor, tomaintain the flexible wing-sail direction parallel to the apparent winddirection.
 18. The wind-propelled vehicle according to claim 17, whereinsaid vehicle further includes a wind velocity sensor for sensing theapparent wind velocity, and a second motor for selectively pivoting theboom to different angles with respect to the mast; and wherein, in saidcontrol system, the manual mode of operation also permits manual controlof said second motor to change the angle of the boom with respect tosaid mast, and thereby to change the curvature of the airfoil shape; andsaid automatic mode of operation automatically controls said secondmotor to change the angle of the boom with respect to the mast, andthereby the curvature of the airfoil shape, in response to the apparentwind velocity as measured by said wind velocity sensor.
 19. A flexiblewing-sail for wind-propelled vehicles, comprising: a mast to berotatably mounted in a vertical position on the vehicle; a plurality offlexible sail panels carried by said mast; and a spreader assemblysecuring said flexible sail panels to the mast and imparting to theflexible sail panels an airfoil shape having a leading edge fore of themast, and a trailing edge aft of the mast; said flexible sail panelsbeing secured to said spreader assembly by battens received in pocketsin said flexible sail panels.
 20. The flexible wing-sail according toclaim 19, wherein said spreader assembly includes a fixed spreader unitsecured to the flexible sail panels defining the bottom of the flexiblewing-sail and fixing it to the bottom of the mast, and a plurality ofslidable spreader units secured to the flexible sail panels atlongitudinally spaced locations thereof and slidable along said mast topermit hoisting and reefing the flexible wing-sail.
 21. The flexiblewing-sail according to claim 19, further comprising: a boom pivotallycoupled at one end to the bottom of said mast and slidably coupled atthe opposite end to the trailing edge of said sail panels by slidesnormally urged rearwardly of the boom by elastic elements; and a brakefor selectively locking the mast against rotation with respect to thevehicle, such that pivoting the boom while the mast is locked againstrotation, changes the curvature of the airfoil shape acording to theapparent wind direction and velocity.